Rainfall/runoff analysis to investigate flowpaths in a forested watershed utilizing topmodel

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SWS-02-1


[W )Soil and Water Science

*r Research BriLef


Rainfall/Runoff Analysis to Investigate Flowpaths in a
Forested Watershed Utilizing Topmodel


Monika Tkaczyk and James Jawitz


Surface runoff or rainfall excess is a
fundamental concept in water resources
planning. The estimate of runoff volume is
of primary concern in engineering design
from urban development to hydraulic
structures and flood prevention.

The rates of hydrologic input and output
processes vary spatially and temporally over
geographic regions at all scales. Data
collection from large areas is difficult and
expensive, so hydrologic models are often
based on homogenous approximations of
nature. As a result, a large basin (1,000
km2) is modeled as a lumped system with
average, representative or "effective"
parameters for the entire area. With the
advancement of computer technology
however, spatially varied parameters like
soil properties, rainfall, surface, topography,
vegetative cover, and land use have become
particularly significant in hydrologic
analyses.

One goal of this study is to analyze rainfall
runoff and subsurface flow in a forested
watershed to predict runoff processes
utilizing spatially varied soil properties. It is
important to be able to model these
processes accurately and estimate rainfall
excess generated from the watersheds, stage
of flooding in a river, or the volume of water
a river can provide at any given time.
Furthermore, surface runoff is of great
interest in water quality issues as it carries
nutrients and contaminants through the
process of erosion, transport and deposition.


The project site is located at Ft. Benning,
Georgia military reservation. Figure 1
illustrates the erosion effects in one of the
watersheds impacted by military training.


A semi-distributed, topography-based
model-TOPMODEL is utilized in this study.
It predicts the flow paths in the watershed
and their effects on the overall watershed
response to precipitation.

The topographic information for the study
area was provided in the form of a digital
elevation map (DEM). This data was
manipulated in the Geographic Information
System (GIS) program ArcView. In
addition, a digital soil properties map was
manipulated in GIS to perform spatially
distributed hydrologic modeling. Figure 2
shows different soil types for the two
watersheds in the study area.






























Sol Map


Sso50 500 1000 Meters


Figure 2. Spatial variability of soil in the study area.

The topographic index (TI) serves as a
primary input information into the model,
upon which the TOPMODEL computes the
hydrologic response of the watershed. This
study introduced a soil-topographic index
(STI), incorporating soil hydraulic
properties into it. Figure 3 compares the TI
with the STI calculated for Bonham 2
watershed. The two curves result in a
different range of topographic index values
and aerial distribution for the two conditions
modeled. As a result, the predicted
saturated areas in the watersheds varied
substantially.

025
Bonham 2 --Top Index
020 So Tond

015

0 10

000


4 6 8 10 12 14 16 18 20 22 24
Top.Index


Figure 3. Topographic index for
heterogeneous soils.


homogenous and


Figure 4 compares TOPMODEL's
simulations: increase in saturated areas for
heterogeneous conditions (Fig. 4b and 4d)
and also a more accurate location of these
areas. A wetland coverage map was
compared to identify wet areas and the
saturated area near the stream, Fig. 4d, was
well depicted by the basin's outlet.


Figure 4. Comparison of the saturated areas for
homogenous run a) and c): heterogeneous run b) and
d).

This research enables improved
management of water and land resources.
Future studies will investigate more closely
the dependence of soil heterogeneity on
hydrograph response and estimated volume
of watershed runoff, as more field data
become available.

Authors:
Monika Tkaczyk, Graduate Research Assist.
James Jawitz, Assistant Professor,
University of Florida,
Soil and Water Science Dept.,
2169 McCarty Hall, P.O. Box 110290,
Gainesville, FL 32611,
monikati@mail.ifas.ufl.edu
jawitziufl.edu